Many modern automobiles utilize a hybrid transmission system in which an internal combustion engine, electric machine, or combination of the two provides propulsion for the vehicle. In a typical hybrid transmission system, torque from the engine and electric machines is supplied to a plurality of gears and shifting elements. Torque is output from this plurality of gears and clutches to drive the wheels of the vehicle. A typical hybrid transmission features either an electrically variable transmission mode (“EVT mode”) or a fixed gear mode, but not both. In an EVT mode, torque must be supplied by both the engine and the electric machines and the overall transmission gear ratio between the transmission input and output can be constantly varied between a theoretically limitless number of gear ratios. In a fixed gear mode, power may be supplied by either or both of the engine and electric machines, but the overall transmission gear ratio between the transmission input and output can only be a limited number of fixed gear ratios. While some hybrid transmissions do feature both an EVT mode and fixed gear mode, these transmissions typically have augmented structures and generally require complicated torque and speed controls.
Typical hybrid transmissions are large and bulky. This makes it more difficult to integrate them into a vehicle's powertrain. Decreasing the size of a typical hybrid transmission often results in decreased torque capability and performance. Further, typical hybrid transmissions serve as the only ratio-adjusting link between the engine and the wheels of the vehicle. This is largely a result of the intricate connections among the engine, electric machine, and hybrid transmission. These complicated connections make the addition of a typical hybrid transmission a complex process. Moreover, these complicated connections typically are not configured as single input and single output and prevent the use of an additional gearbox such as a conventional automatic transmission because of packaging and complexity concerns. This limits the overall gear ratio achievable by the hybrid drive system.
What is needed, therefore, is a compact and robust hybrid drive system that offers both an EVT mode and a fixed gear mode and that retains full functionality of the engine and electric machines regardless of their operating state. What is also needed is a hybrid drive system that achieves an increased range of power modes to expand hybrid power manipulation space and to enhance vehicle drivability. What is further needed is a simple hybrid transmission that can be added to existing automobiles with or without an additional transmission. What is further needed is a hybrid transmission having controls independent from those of an optional automatic transmission.